During a normal heartbeat, the heart contracts in a coordinated fashion to pump blood. In particular, the heart contracts based on rhythmic electrical impulses, which are spread over the heart using specialized fibers. These rhythmic electrical pulses are initiated by the heart's natural pacemaker called the sino-atrial node (“SA node”). The SA node initiates electrical impulses to cause the right and left atria to contract. As the atria contract, the electrical impulses from the SA node propagate to the atrial-ventricular node (“AV node”). The time these impulses take to propagate from the SA node through the AV node is known as the A-V delay. The A-V delay allows the atria to fully contract and fill the ventricles with blood.
The AV node then transmits the impulse, which causes contraction in the right and left ventricles. Blood from the ventricles then flows out of the heart and to the rest of the body. Therefore, the heart relies upon a rhythmic cycle of electrical impulses to pump blood efficiently.
A heart may suffer from one or more cardiac defects that interfere with the rhythmic cycle or conduction of electrical impulses. For example, one known heart condition is an AV block. An AV block inhibits transfer of impulses from the SA node to the AV node, and thus, inhibits or prevents contraction of the right and left ventricles. Other conditions, such as myocardial scarring and bundle branch block, may slow conduction of impulses, and thus, cause the heart to beat in an uncoordinated fashion.
Typically, an artificial pacemaker is installed to treat these cardiac deficiencies. The artificial pacemaker senses impulses from the SA node and then supplies stimulating electrical pulses to cause contraction in chambers of the heart, such as the ventricles. Therefore, an artificial pacemaker may compensate for blocked or slowed conduction of electrical impulses in the heart.
The specialized cardiac fibers in the heart are completely surrounded by a cell membrane. In a given chamber of the heart, at the points where the ends of the individual fibers meet, two individual cell membranes fuse into a single structure. These structures are known as intercalated discs and they provide a strong connection among all of the individual fibers of the heart. Intercalated discs provide bridges of low electrical resistance, and thus, allow for the rapid propagation of electrical signals throughout the heart during contraction. This phenomenon is known as a functional synctium.
Since adjacent cardiac fibers in a chamber of the heart normally form a functional synctium, known artificial pacemakers include only a single electrode in each chamber. Known artificial pacemakers thus rely on the functional synctium to propagate a stimulating electrical pulse throughout a chamber, even though the stimulus originates from a single electrode.
However, there are cardiac deficiencies that may interfere with the proper contraction within a particular chamber. For example, a chamber may suffer from a defect or injury that blocks the propagation of electrical impulses within the chamber or prevents a portion of the chamber from contracting in a coordinated fashion with other chambers of the heart. As another example, patients with congestive heart failure (CHF) may experience sufficient asynchrony within a single chamber of the heart that the chamber is unable to properly pump blood within a normal rhythmic cycle.
Previously, treatment by stimulating right and left ventricles at the same or similar times has assisted in treating asynchrony. Unfortunately, known artificial pacemakers cannot completely compensate for asynchrony within a single chamber. As noted above, known artificial pacemakers only apply stimulating pulses to a single location within a given chamber using only one electrode. Accordingly, it would be desirable to provide methods, apparatus, and systems, which can overcome these and other deficiencies in the prior art, for example, to assist any given chamber of the heart to contract in a much more coordinated fashion, and thus, assist the heart in contracting more efficiently as a whole in a coordinated fashion. In addition, it would be desirable to provide methods, apparatus, and systems, which can stimulate multiple sites in a chamber of the heart, such as the left ventricle.